Plant genes that encode proteins similar to mammalian glutamate receptors, which are critical to neuronal function, were identified some time ago, but the function of these proteins in plants has been a mystery. Now, Michard et al. examined two Arabidopsis glutamate receptor–like channels (GLRs) that are expressed in pollen and affect pollen tube growth. The unusual amino acid D-Ser produced by target female tissues activated GLRs in the apical region of pollen tubes, allowing Ca2+ permeation into the cytoplasm, and promoted pollen tube growth. This modulation of function by an amino acid signal suggests how pollen tube growth may be coordinated with signals from the tissues that they must grow through.